Both of poly-ε-caprolactone (PCL) and polylactide (PLA) are biodegradable material of polyesters and have excellent biodegradability, thus they are widely used in producing disposable containers, tableware, thin films, fibers, clothes and automotive elements, which is of benefit to solve the environment pollution problem caused by petroleum chemical resources, Furthermore, PCL and PLA also have properties such as good biocompatibility, nontoxicity and adjustable degradability, thus they meet the requirements of high molecular materials useful in medicine and have a prospect for use in internal/external fixation of fracture, surgical suture, stent in tissue engineering and sustained release of drugs, release-controlling support materials and the like.
The earlier synthesis of polyesters is mainly the condensation reaction of acids and alcohols. However, the polymer structure synthesized by such reaction cannot be controlled, and possibly is a linear, branched or cyclic structure, and the molecular weight distribution thereof is too wide, the molecular weight thereof is low and is hard to be controlled, so that the mechanic properties of the polymer are poor eventually. In recent years, researches on the synthesis of polyesters are mainly focused on developing the coordination polymerization catalyst to initiate the ring opening of cyclic esters to produce polyester polymers. Compared with the direct dehydration polycondensation method, the ring opening polymerization method for producing polyesters has the following method, the ring opening polymerization method for producing polyesters has the following advantages: (1) the molecular weight of polyesters can be controlled precisely, and the molecular weight distribution is very narrow; (2) the polyesters obtained by the direct dehydration condensation have a low molecular weight and the properties thereof cannot satisfy certain requirements in biomedicine, while no any water is produces in the ring opening polymerization and thus polymers having a higher molecular weight can be synthesized; (3) selective polymerization of chiral monomers can be achieved by modifications on the catalyst ligands.
The catalyst systems applied in the coordination ring opening polymerization of lactones mainly include stannous octoate, the complexes of aluminum, calcium, magnesium, zinc and titanium, the complexes of metals of group IIIB, and the like. For example, the Chinese patent document with the application number 01114244.8 discloses a method for producing poly-ε-caprolactone by using stannous octoate as the catalyst. However, the molecular weight of the poly-ε-caprolactone obtained by the catalysis with stannous octoate cannot be controlled, and the molecular weight distribution thereof is wide. The Chinese patent document with the application number 200610016623.3 discloses a production method and use method of a Schiff base aluminum catalyst for the ring opening polymerization of lactide. However, the catalytic activity of the Schiff base aluminum is low, and the molecular weight of the polylactide obtained eventually is relatively low.